As global warming and environmental problems become serious, electric cars or hybrid electric cars are actively developed as clean automobiles replacing gasoline cars. Energy storage devices used for such applications are required to achieve both high energy density and high output characteristics, and at the same time, a durability of more than 10 years, high safety, and the like are required of the energy storage devices.
In order to achieve both high energy density and high output density, it is effective to increase operating voltage. Therefore, a negative electrode comprising a substance which lithium ions can be reversibly intercalated into and deintercalated from, and an electrolytic solution comprising an aprotic organic solvent comprising a lithium salt are used for these energy storage devices.
Examples of typical positive electrode materials used in these energy storage devices include transition metal oxides. This energy storage device is called a lithium ion secondary battery and is characterized by having very high energy density. In recent years, the output characteristics have also been improved, and both improved output characteristics and high energy density have been achieved. However, the problem of a decrease in safety due to thermal runaway, the problem of a rise in price due to the shortage of resources, the problem of environmental load, and the like remain, and the energy storage device has not been widely put to practical use for automobiles.
Examples of another positive electrode material replacing transition metal oxides include activated carbon. These energy storage devices using activated carbon for the positive electrode are called lithium ion capacitors. Since the energy storage device stores a charge by the electrostatic mechanism of an electric double layer, the energy storage device is characterized in that although the energy density is small, the output density is high, and the cycle stability is also high. There are no problems of resources and safety as in transition metal oxides. The technique of predoping the negative electrode with lithium ions by a chemical method or an electrochemical method is used to increase capacity, but still sufficient capacity is not obtained, and the energy storage device has not been widely put to practical use for automobiles so far (for example, see Patent Literatures 1 and 2).
In addition, a nitroxyl compound having an oxoammonium cation partial structure in an oxidized state and having a nitroxyl radical partial structure in a reduced state is proposed as another positive electrode material replacing transition metal oxides. This energy storage device is called an organic radical secondary battery and is known as a battery that exhibits high output characteristics, has small environmental load, and is safe. But, sufficient cycle stability is not obtained, and therefore, the energy storage device has not been widely put to practical use so far, and further improvement is desired (for example, see Patent Literature 3).
As described above, it has been difficult so far to fabricate an energy storage device that satisfies sufficiently high energy density and output density, low environmental load, and cycle stability, as an energy storage device that can be widely put to practical use for automobiles.